The biosynthesis of γ-substituted glutamic acids in Gleditsia triacanthos

Abstract

The concentrations of several glutamic acid derivatives, each having a γ-methylene or γ-methyl substituent, increased rapidly during the early stages of germination and growth of Gleditsia triacanthos seeds. Gleditsia seedlings therefore formed a suitable system for biogenetic studies with this group of amino acids. Various 14C-labelled substances were supplied to seedlings and, after 2 or 4 days metabolism, the incorporation of radioactivity into individual substituted glutamic acids was determined. These experiments showed that the branched C 6 skeleton was unlikely to arise directly by the addition of a C 1 unit (from methionine) at the γ-carbon atom of glutamic acid. In contrast, the seedlings converted [1- 14C]leucine into erythro-γ-methylglutamic acid, in which the 14C label was located essentially in the α-carboxy C atom, i.e. in the expected position for a direct pathway converting a terminal methyl group of leucine into a γ-carboxyl group by oxidative processes. 2(S),4(S)-γ-hydroxy-γ-methylglutamic acid was probably formed by direct hydroxylation of the parent acid, erythro-γ-methylglutamic acid. When [1- 14C]pyruvate was supplied to seedlings, considerable amounts of radioactivity were incorporated into the γ-substituted glutamic acids. However, only very small percentages of the 14C were present in the α-carboxyl group of the C 6 amino acids, so the idea that the branched carbon skeleton arises by condensation of two molecules of pyruvate (or some activated derivative) is untenable.